Molecular Orientation of Evaporated Pentacene Films on Gold

Jie Ling Lou , Hung Wei Shiu , Lo Yueh Chang , Chia Ping Wu , Yun-Liang Soo ..... Yuseong Gim , Boseok Kang , BongSoo Kim , Sun-Guk Kim , Joong-Hee Le...
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Molecular Orientation of Evaporated Pentacene Films on Gold: Alignment Effect of Self-Assembled Monolayer W. S. Hu,‡ Y. T. Tao,*,†,‡ Y. J. Hsu,§ D. H. Wei,§ and Y. S. Wu§ Institute of Chemistry, Academia Sinica, Taipei,115, Taiwan, R.O.C., Department of Chemistry, National Tsing-Hua University, Hsinchu, 300, Taiwan, R.O.C., and National Synchrotron Radiation Research Center, Hsinchu, 300, Taiwan, R.O.C. Received September 23, 2004. In Final Form: December 31, 2004 Pentacene films deposited on self-assembled monolayers (SAMs) bearing different terminal functional groups have been studied by reflection-absorption IR, grazing angle XRD, NEXAFS, AFM, and SEM analyses. A film with pentacene molecules nearly perpendicularly oriented was observed on Au surfaces covered with an SAM of alkanethiol derivative of X-(CH2)n-SH, with X ) -CH3, -COOH, -OH, -CN, -NH2, C60, or an aromatic thiol p-terphenylmethanethiol. On the other hand, a film with the pentacene molecular plane nearly parallel to the substrate surface was found on bare Au surface. A similar molecular orientation was found in thinner (∼5 nm) and thicker (100 nm) deposited films. Films deposited on different surfaces exhibit distinct morphologies: with apparently smaller and rod-shaped grains on clean bare Au surface but larger and islandlike crystals on SAM-modified surfaces. X-ray photoemission electron microscopy (X-PEEM) was used to analyze the orientation of pentacene molecules deposited on a SAM-patterned Au surface. With the micro-NEXAFS spectra and PEEM image analysis, the microarea-selective orientation control on Au was characterized. The ability to control the packing orientation in organic molecular crystals is of great interest in fabricating organic field effect transistors because of the anisotropic nature of charge transport in organic semiconducting materials.

Introduction Electronic devices based on organic semiconducting materials have already been demonstrated in areas such as light-emitting diodes, rectifiers, and solar cells.1 Organic semiconducting materials offer the advantages of low cost and low-temperature processing when compared with silicon-based materials. Field effect transistors (FETs) based on organic semiconducting materials have also made rapid progress in recent years. Many conjugate molecules have been suggested to possess high enough charge mobility. Among these, pentacene is one of the best candidates for use in fabricating thin film transistors because of their high field effect charge mobility exhibited in the single crystal state.2,3 The ordered molecular packing in a crystal is crucial for high mobility because of fewer trap sites present in the crystals. Furthermore, it is widely accepted now that the carrier mobility in organic FETs is determined by the degree of π-orbital overlap between adjacent conjugate molecules, which facilitate the charge-hopping process.4-6 A more efficient device is expected if all the molecules are packed orderly with the conducting channels in correct alignment * Corresponding author. Telephone: +886-2-27898580. Fax: +886-2-27831237. E-mail: [email protected]. † Academia Sinica. ‡ National Tsing-Hua University. § National Synchrotron Radiation Research Center. (1) Kelley, T. W.; Baude, P. F.; Gerlach, C.; Ender, D. E.; Muyres, D.; Haase, M. A.; Vogel, D. E.; Theiss, S. D. Chem. Mater. 2004, 16, 4413. (2) Dimitrakopoulos, C. D.; Malenfant, P. R. L. Adv. Mater. 2002, 14, 99. (3) Sundar, V. C.; Zaumseil, J.; Podzorov, V.; Menard, E.; Willett, R. L.; Someya, T.; Gershenson, M. E.; Rogers, J. A. Science 2004, 303, 1644. (4) Loiacono, M. J.; Granstrom, E. L.; Frisbie C. D. J. Phys. Chem. B 1998, 102, 1679. (5) van de Craats, A. M.; Stutzmann, N.; Bunk, O.; Nielsen, M. M.; Watson, M.; Mu¨llen, K.; Chanzy, H. D.; Sirringhaus, H.; Friend, R. H. Adv. Mater. 2003, 15, 495. (6) Bre´das, J. L.; Calbert, J. P.; da Silva Filho, D. A.; Cornil, J. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5804.

relative to the electrodes, which means that the molecules should be aligned with their molecular planes perpendicular or parallel to the substrate depending on the configuration of the FET. Thus aligning the molecular crystals relative to the source/drain is important for an efficient device. There are a number of approaches to affect the orientation of a molecular moiety on a surface. These include surface modification,7 mechanical rubbing,8 and substrate temperature control.9 There are many studies on the oriented growth of pentacene films on various substrates, which include graphite,10 Ag(111),11 Cu(110),12,13 Au(111),14 SnS2,15 and SiO2.16 The orientation of the first deposited pentacene monolayer is strongly influenced by the substrate properties. A flat-lying pentacene monolayer has been observed on Cu(110)13 and Au(111).14 The electronic interaction between π-orbitals of pentacene and empty d-orbitals of the metal atoms may cause the initial flat-lying pentacene monolayer.17 On the other hand, a monolayer of pentacene adopts a perpendicular orientation relative to the SiO2 surface.18 The n-alkanethiol and its ω-functional derivatives are known to form closely packed monolayers on gold surface,19 with the linear alkyl chains extended trans zigzag with (7) Naciri, J.; Fang, J. Y.; Moore, M.; Shenoy, D.; Dulcey, C. S.; Shashidhar, R. Chem. Mater. 2000, 12, 3288. (8) Wittmann, J. C.; Smith, P. Nature 1991, 352, 414. (9) Resel, R.; Koch, N.; Meghdadi, F.; Leising, G.; Unzog, W.; Reichmann, K. Thin Solid Films 1997, 305, 232. (10) Harada, Y.; Ozaki H.; Ohno, K. Phys. Rev. Lett. 1984, 52, 2269. (11) Casalis, L.; Danisman, M. F.; Nickel, B.; Bracco, G.; Toccoli, T.; Iannotta, S.; Scoles, G. Phys. Rev. Lett. 2003, 90, 206101. (12) Lukas, S.; So¨hnchen, S.; Witte, G.; Wo¨ll, C. Chem. Phys. Chem. 2004, 5, 266. (13) Lukas, S.; Witte, G.; Wo¨ll, C. Phys. Rev. Lett. 2002, 88, 028301. (14) Kang, J. H.; Zhu, X. Y. Appl. Phys. Lett. 2003, 82, 3248. (15) Schroeder, P. G.; France, C. B.; Park, J. B.; Parkinson, B. A. J. Appl. Phys. 2002, 91, 3010. (16) Gundlach, D. J.; Lin, Y. Y.; Jackson, T. N.; Nelson S. F.; Sclom, D. G. IEEE Electron Device Lett. 1997, 18, 87. (17) Yamamoto, T. Synlett 2003, 4, 425. (18) Fritz, S. E.; Martin, S. M.; Frisbie, C. D.; Ward, M. D.; Toney, M. F. J. Am. Chem. Soc. 2004, 126, 4084.

10.1021/la047634u CCC: $30.25 © 2005 American Chemical Society Published on Web 02/18/2005

Molecular Orientation of Pentacene Films on Gold

a tilt angle of ∼30° from the surface normal and exposing the terminal group at the substrate/air interface.20 These self-assembled monolayers (SAMs) provide a systematic way to modify surface properties such as wetting and polarity of a substrate, which, in turn, may affect the orientation of a film deposited on top of it.21 Recently, an SAM of organosilane on SiO2 was reported to affect the carrier density of the pentacene channel material through the dipole of a terminal functional group.22 However, the detailed structure of the pentacene film deposited on SAMs with different terminal functional groups was not addressed, which may also affect the overall property. In this work we used various SAMs prepared on a thermally evaporated Au surface and examined the molecular orientation of pentacene crystals thermally deposited on the SAM surfaces. It was found that the SAMs exposing a nonpolar methyl group, an aromatic p-terphenyl group, or polar groups such as carboxyl, hydroxyl, cyano, and amino groups all induced perpendicular alignment of pentacene moieties, in contrast to the parallel alignment found on a bare gold surface. The oriented growth of pentacene crystal film extends to a thickness of at least 100 nm. An orientationally patterned pentacene film was obtained on a Au surface patterned with an SAM, due to a different alignment effect of the underlying surface, as evidenced by the photoemission electron microscopy (PEEM) technique.23-26 Experimental Section The gold substrate was prepared by evaporation of high-purity (99.99%) gold from a resistively heated tungsten boat onto a silicon (100) wafer, which was precleaned by a Piranha solution (H2SO4:H2O2 ) 4:1). To increase the adhesion between gold and silicon wafer, a 130 Å chromium film was deposited on the silicon wafer prior to the deposition of 130 nm of gold. All the organic thiols X(CH2)nSH (with X ) -COOH, -OH, -CN, n ) 16; with X ) -NH2, n )11) and the aromatic thiol p-terphenylmethanethiol were prepared according to literature procedures.20,27 The monolayers were formed by soaking in a dilute (1 mM) solution of X(CH2)nSH in ethanol for 24 h.20 The p-terphenylmethanethiolmodified surface was prepared by immersing the Au substrate in a mixed solution (2 mL of THF and 8 mL of EtOH) containing 1 mM p-terphenylmethanethiol for 1 h. The C60-modified surface was prepared from the NH2-terminated monolayer by treating the surface with a solution of C60 in benzene.28 The resulting substrate was thoroughly rinsed with benzene to remove residual physisorbed C60. Commercially obtained pentacene (Aldrich Chemical Co.) was purified by subliming twice through a temperature-gradient sublimator. The deposition of pentacene was carried out at room temperature in a vacuum chamber at an evaporation rate of 1 Å/s under a pressure of 2 × 10-5 Torr. (19) Ulman, A. An Introduction to Ultrathin Organic Films, From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (20) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (21) Shtein, M.; Mapel, J.; Benziger, J. B.; Forrest, S. R. Appl. Phys. Lett. 2002, 81, 268. (22) Kobayashi, S.; Nishikawa, T.; Takenobu, T.; Mori, S.; Shimoda, T.; Mitani, T.; Shimotani, H.; Yoshimoto, N.; Ogawa, S.; Iwasa, Y. Nature Mater. 2004, 3, 317. (23) Sto¨hr, J.; Wu, Y.; Hermsmeier, B. D.; Samant, M. G.; Harp, G. R.; Koranda, S.; Dunham, D.; Tonner, B. P. Science 1993, 259, 658. (24) Nolting, F.; Scholl, A.; Sto¨hr, J.; Seo, J. W.; Fompeyrine, J.; Siegwart, H.; Locquet, J. P.; Anders, S.; Lu¨ning, J.; Fullerton, E. E.; Toney, M. F.; Scheinfein, M. R.; Padmore, H. A. Nature 2000, 405, 767. (25) Cossy-Favre, A.; Diaz, J.; Liu, Y.; Brown, H. R.; Samant, M. G.; Sto¨hr, J.; Hanna, A. J.; Anders, S.; Russel, T. P. Macromolecules 1998, 31, 4957. (26) Ade, H.; Winesett, D. A.; Smith, A. P.; Anders, S.; Stammler, T.; Heske, C.; Slep, D.; Rafailovich, M. H.; Sokolov, J.; Sto¨hr, J. Appl. Phys. Lett. 1998, 73, 3775. (27) Himmel, H.; Terfort, A.; Wo¨ll, C. J. Am. Chem. Soc. 1998, 120, 12069. (28) Caldwell, W. B.; Chen, K.; Mirkin, C. A.; Babinec, S. J. Langmuir 1993, 9, 1945.

Langmuir, Vol. 21, No. 6, 2005 2261 The bare gold surface was cleaned by an UV-ozone generator for 12 h, followed by deionized water rinse. Before loading the substrate into the deposition chamber, additional cleaning by a low-power oxygen plasma for 5 min was carried out to ensure the surface cleanliness. The film thickness was monitored by quartz crystal microbalance. For systematic comparison, the deposition was always done on a pair of substrates, one with SAM modification and one without. The hard poly(dimethylsiloxane) (h-PDMS) stamp for microcontact printing was prepared according to literature procedures.29,30 The stamp was inked with an ethanolic solution of 1 mM thiol derivative and brought into contact with the gold surface for 2 min, and then the resulting substrate was thoroughly rinsed in pure EtOH and dried with a stream of nitrogen. Reflection-absorption IR spectra were recorded in single reflection mode with a grazing incidence angle of 86°, using a Digilab FTS 60A Fourier transform infrared spectrometer (BioRad, Cambridge, MA). A liquid nitrogen cooled MCT detector was used. Four hundred scans were collected for spectral data processing. X-ray diffraction patterns were obtained at the wiggler 17B beamline of the National Synchrotron Radiation Research Center (NSRRC), Taiwan. A grazing incidence angle below the critical angle of the total reflection from the substrate and above the critical angle for the pentacene film was chosen to enhance the sensitivity of measurement. AFM measurements were carried out under ambient conditions using a Digital Instruments Nanoscope III microscope operated in the tapping mode. The SEM analysis was carried out on a Hitachi electron microscope (SEM Hitachi S-4200), with 15 keV cold field emission scanning. The near-edge X-ray absorption fine structure (NEXAFS) spectra were taken at the X-ray photoemission electron microscopy (X-PEEM) station at NSRRC. s- and p-polarized radiation with an energy resolution of 100 meV at the carbon K-edge and a focused spot size of 0.1 × 1 mm2 was delivered to the sample at 65° incident angle through an elliptically polarized undulator (EPU5.6) beamline. The sample current was measured with a picoammeter to obtain the NEXAFS spectra in the total electron yield (TEY) mode. All the raw NEXAFS data were normalized to the photoelectron yield of a clean gold mesh. The energy was calibrated with the intense π* resonance of graphite at 285.4 eV. For PEEM imaging, the photoemitted electrons were accelerated into the electron optics with 5 kV accelerating voltage. A data acquisition system equipped with a Photometrix Quantix CCD camera was used to record the images. The images were taken at different excitation energies and normalized by subtracting the preedge image. To minimize the radiation damage and heavy carbon contamination, a fresh sample area was exposed to the X-ray beam spot for each data collection.

Results and Discussion Reflection-Absorption IR. Reflection-absorption IR spectroscopy provides useful information in molecular orientation on a reflective surface based on the selection rule that only vibrational modes having a transition dipole along the surface normal will get excited.31 Parts a and b of Figure 1 compare the reflection-absorption IR spectra of a 5-nm-thick pentacene film deposited on a bare gold surface and a p-terphenylmethanethiol-modified gold surface. The contribution of the monolayer to the spectra has been subtracted so that the spectra reflect only the characteristics from the pentacene film. The most prominent features in the spectrum for bare gold surface are the strong absorptions at 731 and 907 cm-1, respectively, which are assigned to the out-of-plane bending vibrations of the aromatic ring C-H’s. These vibration modes have their transition dipoles align normal to the ring plane. The vibration modes associated with the ring stretches (29) Scmid, H.; Michel, B. Macromolecules 2000, 33, 3042. (30) Odom, T. W.; Christopher Love, J.; Wolfe, D. B.; Paul, K. E.; Whitesides, G. M. Langmuir 2002, 18, 5314. (31) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 96, 927.

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Figure 2. Reflection-absorption IR of 5-nm-thick pentacene film deposited on SAMs with different terminal functional groups.

Figure 1. Reflection-absorption IR of 5-nm-thick pentacene film deposited on (a) bare gold and (b) SAM of p-terphenylmethanethiol on gold.

and C-H stretches, which have the transition dipole parallel to the ring plane, are much weaker. For 5-nmthick pentacene film deposited on the p-terphenylmethanethiol-modified surface, the spectra (Figure 1b) are very different. Significant absorptions were observed in the range above 3000 cm-1, which are assigned to the ring C-H stretch vibrations. Also apparent are the ring stretch modes appearing around 1500 cm-1. On the other hand, the out-of-plane vibration modes at 907 and 731 cm-1 are greatly reduced. These observations strongly suggest different orientations of the pentacene molecules in the film on different surfaces: lying flat on bare gold and more or less perpendicular to the substrate on the SAM-modified gold. The effect of various terminal functional groups was also examined. IR patterns similar to that obtained on p-terphenylmethanethiol-modified gold were found on substrates with SAMs bearing terminal -CN, -CH3, -COOH, -NH2, -OH, or -C60 groups (Figure 2). Thus the orientation of pentacene in the film was similar, irrespective of the polarity or character of the terminal functional group used. Additional characterizations were described mainly for the films deposited on the aromatic thiol SAM system as a representative. NEXAFS Spectroscopy. For a pentacene film with well-ordered and oriented packing, the NEXAFS spectrum, which probes the excitation of core electrons to the upper empty orbitals, will give a strong dichroism for s-polarized and p-polarized incident X-rays. The polarization-dependent K-shell spectra as well as their difference spectra (∆) for a 4.5-nm-thick film of pentacene deposited on the

Figure 3. Carbon K-edge absorption spectra of 4.5-nm-thick pentacene film deposited on (a) bare Au surface and (b) p-terphenylmethanethiol-modified Au surface.

bare gold surface are shown in Figure 3a. A much enhanced signal in the π*-transition region (280-287 eV) of the carbon K-edge NEXAFS is found for p-polarization, which has the E vector perpendicular to the substrate, as compared with the case of s-polarization, which has the E vector parallel to the substrate. Features in the region 287-291 eV are assigned to be the C-H*/Rydberg resonances, whereas the broad band appearing in the region 291-315 eV are characterized as σ* resonances. The core to σ* resonance is much enhanced in the s-polarization. This suggests that the aromatic plane of the pentacene is lying flat on the bare gold surface, so that the empty π*-orbital is aligned nearly perpendicular to the surface. With this orientation, the σ*-orbital is aligned parallel to the surface, giving an enhanced peak for the s-polarization. In contrast, for the film deposited

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Figure 4. Carbon K-edge absorption spectra of 4.5-nm-thick pentacene film deposited on various SAM-covered Au surfaces with (a) p-polarized and (b) s-polarized irradiation.

Figure 5. Polarization-dependent carbon K-edge NEXAFS spectra of 100-nm-thick pentacene film on (a) bare gold surface and (b) p-terphenylmethanethiol-modified gold surface.

on p-terphenylmethanethiol-modified gold surface, the spectrum gives a stronger intensity of the π*-transition for s-polarized X-ray than for p-polarized X-ray (Figure 3b).32 This is opposite what is observed for the film deposited on the bare gold surface. The pentacene moiety in the film is suggested to align perpendicular to the SAMmodified surface. Figure 4 shows the NEXAFS measurements for a 4.5-nm-thick pentacene film deposited on several SAM-covered surfaces. When comparing the NEXAFS spectra for pentacene films on phenyl-, methyl-, and carboxyl-terminated thiols, virtually identical spectra were obtained, indicating that similar orientation of the pentacene molecules is adopted on these surfaces, a result in agreement with IR observations. Thus the polarity of the terminal functional group in the SAM does not exert a different effect in aligning the molecules. A comparison of the NEXAFS spectra obtained for a thicker (100 nm, Figure 5) film and the thinner (4.5 nm) film and films of other thicknesses, shows that a very similar dichroism is found. The morphology is also similar for thinner films and thicker films on the same surface (vide infra). As the (32) Comparing the NEXAFS spectra of the p-terphenylmethanethiol SAM and the one with pentacene film deposited on top, it is concluded that the contribution of monolayer to the overall NEXAFS spectrum is negligible, presumably due to the probe depth of NEXAFS; see ref 33.

probe depth for NEXAFS is about 2-10 nm,33 these observations would suggest that the orientation order extends over a wide range of thickness. It is proposed that the type of surface influences the orientation of initial molecules at the surface, which in turn influence the orientation of the following molecules. This is in agreement with an earlier report that the orientation is independent of the thickness,14 but in contrast to a recent suggestion34 that a change of flat-lying orientation to vertical orientation occurred at thicknesses beyond ∼5 nm. It is noted that the later report used organic molecular beam deposition (OMBD) on single crystal gold at very slow deposition rate (0.5-3 Å/min) to prepare the film sample, whereas our work used vacuum sublimation with a higher deposition rate of ∼1 Å/s to deposit the films. Assuming that the light is ideally polarized, then the correlation Is/Ip ) (1/2) tan2 R applies, where I is the intensity of the π* resonance and R is the tilt angle between the molecular plane and the surface.35,36 The intensity ratio of π* at 285 eV in Figure 5a implies an average (33) Sto¨hr, J.; Anders, S. IBM J. Res. Dev. 2000, 44, 535. (34) Beernink, G.; Strunskus, T.; Witte, G.; Wo¨ll, C. Appl. Phys. Lett. 2004, 85, 398. (35) Sto¨hr, J. NEXAFS Spectroscopy; Springer: Berlin, 1991. (36) Mainka, C.; Bagus, P. S.; Schertel, A.; Strunkus, T.; Grunze, M.; Wo¨ll, C. Surf. Sci. 1995, 341, 1055.

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Figure 6. X-ray diffraction pattern of pentacene films deposited on different surfaces: (a) typical θ-2θ scan and (b) grazing incidence X-ray diffraction.

inclination angle of 47.5° for the ring plane. This result can be explained by the herringbone packing structure of pentacene molecules within the crystals on the bare Au surface. Figure 5b shows the polarization-dependent carbon K-edge NEXAFS spectra for a 100-nm-thick pentacene film deposited on the p-terphenylmethanethiolmodified Au surface. An average inclination angle of 67.9° from the surface (or 22.1° from the surface normal) is calculated for the long axis of pentacene on p-terphenylmethanethiol SAM surface. X-ray Diffraction Study. Further evidence on the orientation and packing phases comes from an X-ray diffraction study. Pentacene was reported to crystallize in different morphologies characterized by their d (001) spacings of 14.1, 14.5, 15.0, and 15.3 Å. Such polymorphism is often observed in molecular crystals prepared under different conditions such as pressure, substrate temperature, substrate character, carrier gas flow rate, and other variables. X-ray diffraction was carried out on a 100-nmthick pentacene sample deposited on bare gold and a p-terphenylmethanethiol-covered gold substrate (Figure 6). A control experiment showed that the monolayer does not give detectable diffraction patterns under present conditions; thus the diffraction patterns observed for films deposited on an SAM are assumed to be due to the films only. In the film deposited on the SAM-modified gold, diffraction peaks assignable to (001), (002), (003), and (004) can be found, with a corresponding interplanar d spacing of 15.3 Å for (001). These diffraction peaks suggest that pentacene film on the SAM surface has a thin film phase like crystal structure.2 It also suggests that the long axis

Figure 7. AFM micrographs of 50-nm-thick pentacene films deposited on (a) bare gold surface and (b) p-terphenylmethanethiol-modified gold surface.

of pentacene (16.01 Å) aligns at an angle of about 18° from the surface normal, which is in reasonable agreement with the tilt calculated from NEXAFS spectra (∼22°). In contrast, for the film deposited on a bare gold surface, diffraction peaks corresponding to much lower d spacings, ∼4.6 Å at 19.24°, ∼3.7 Å at 23.88°, and ∼3.15 Å at 28.28°, were observed, implying the pentacene is lying flat on the surface with an interplanar spacing around 3 Å. The grazing incidence X-ray diffraction pattern (Figure 6b) was also taken. Two different orientations of the polycrystalline pentacene film with respect to the substrate have been identified. The pentacene film on Au surface exhibits a strong Bragg reflection at 6.16° in a grazing angle XRD experiment which is close to (001) of the bulk phase pentacene crystal and lies in the Au surface plane.37 On the other hand, the pentacene film on SAM-modified Au surface exhibits a strong diffraction peak at 19.36° in grazing angle XRD measurement, which is likely to be from the (110) reflection of pentacene. In contrast, no Bragg reflection peak was observed at 6.16° in this sample. These results indicate that there is remarkably different crystal orientation control by modifying the Au surface with an organic SAM. (37) Campbell, R. B.; Trotter, J.; Robertson, J. M. Acta Crystallogr. 1962, 15, 289.

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Figure 10. Background-subtracted PEEM image with spolarization.

Figure 8. SEM micrographs of 100-nm-thick pentacene films deposited on (a) bare gold surface and (b) p-terphenylmethanethiol-modified gold surface.

Figure 9. PEEM images of 5-nm-thick pentacene film on p-terphenylmethanethiol-patterned Au surface taken with (a) p-polarized and (b) s-polarized X-ray at 278, 285, and 292 eV, respectively. The field of view (diameter of the whole image) is 70 µm.

Film Morphology. The AFM images of 50-nm-thick films deposited on the SAM-modified surface and the bare gold surface at room temperature are shown in Figure 7. For film deposited on a bare gold surface, the grains are apparently smaller and rod-shaped. A surface roughness

of ∼12 nm was observed. However, on the p-terphenylmethanethiol-modified surface, the crystals are in general larger and appear to expose different faces of the crystals. The surface is smoother, with an average roughness of 7 nm. The different molecular packing arrangement in the organic molecular crystal also results in different apparent morphologies. A previous work showed that a monolayer of deposited pentacene on Au surface gave a wandering streamlike pattern. These patterns may induce the formation of rodlike pentacene nanocrystals observed here.15 Figure 8 shows the scanning electron micrographs of 100-nm-thick pentacene films deposited on the p-terphenylmethanethiol monolayer surface and bare gold surface at room temperature. The observed morphological results were consistent with the previous AFM analysis. For the film deposited on bare gold surface, the grains are apparently smaller and rod-shaped. The long axis of the rod is randomly oriented in the substrate plane. Some of the rodlike crystals appear in a bent shape. The films deposited on SAM surface have larger grain sizes. X-PEEM and Micro-NEXAFS Analysis. In view of the different effects on the molecular packing in pentacene film by a bare gold and a SAM-covered gold surface, a gold surface micropatterned with a p-terphenylmethanethiol SAM by microcontact printing technique29,30 is used as the substrate. The pentacene thin films deposited on the SAM-patterned Au surface were analyzed by X-ray photoemission electron microscopy (X-PEEM). The ppolarized X-ray PEEM images taken at photon energies of 278, 285, and 292 eV of a 5-nm-thick pentacene film deposited on p-terphenylmethanethiol SAM-patterned gold are shown in Figure 9a. The image has a field of view of 70 µm. The dark areas in the micrograph are areas where a monolayer of p-terphenylmethanethiol was adsorbed. These SAM-modified areas are visible at the photon energies 278 and 285 eV, but the image contrast becomes weak at 292 eV. The weak contrast at 292 eV is due to the enhanced electron yield from the σ* transition of pentacene molecules standing nearly perpendicular in the SAM-modified area under illumination of ppolarized incident X-rays. The s-polarized X-ray PEEM images of this sample are shown in Figure 9b, also taken at photon energies of 278, 285, and 292 eV, respectively. The image contrast becomes blurred with an incidence photon energy at 285 eV. The result can be explained by the large electron yield from the pentacene film on SAM-modified areas due to the π* transition at 285 eV. The contrast is further enhanced after subtraction of the preedge image (the background taken at 278 eV) from the absorption edge image, the result of which is attributable to the π* transition only (Figure 10). This reversed contrast confirms that the lighted areas are mainly due to the π* absorption of vertically aligned pentacenes rather than a topography contrast. Figure 11a shows p-polarized micro-NEXAFS spectra for film deposited on the SAM patterned gold surface. The sampling area for generating the micro-NEXAFS spectra is

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Figure 11. Micro-NEXAFS spectra of selected areas for a 5-nm-thick pentacene film deposited on p-terphenylmethanethiolpatterned Au surface; spectra taken with (a) p-polarized X-ray and (b) s-polarized X-ray. The sampled areas are marked as a red circle for pentacene on SAM and a black triangle for pentacene on Au in Figure 10.

10 µm × 10 µm (marked in Figure 10). The π* resonance of pentacene molecules on Au (dark image area in Figure 10) is enhanced by p-polarized X-rays. In contrast, the π* resonance is enhanced by s-polarized X-rays for film on the SAM-modified gold surface (bright image area in Figure 10), as shown in Figure 11b. The polarizationdependent micro-NEXAFS spectra in these selected microareas show features similar to that of the pentacene film deposited on Au and a SAM-modified surface, respectively. Thus the influence of the substrate on the orientation of the pentacene film deposited on it maintains in micrometer-scale patterns. Conclusion In conclusion, we have demonstrated that self-assembled monolayers of various terminal functional groups exert similar effects in aligning the pentacene molecules perpendicular to the surface compared with bare gold substrate, where a parallel orientation was found. A distinct morphology was also observed on these two surfaces. The parallel alignment may be attributed to the favorable interaction between the π cloud of pentacene and the empty d-orbital of the metal. Such an interaction is blocked in the presence of a monolayer. Instead, strong intermolecular π stacking dominates in the nucleation

process of the film forming. The orientation effect was retained for a thicker film because of the intermolecular interaction. The monolayer-modified Au surface also favors a larger grain formation presumably due to a higher mobility of molecules on the monolayer surface, which presents a surface of low critical surface tension of wetting. We also successfully demonstrated the area-selective molecular orientation packing control by use of the microcontact printing technique to form a template for area-selective alignment. This is of great interest for fabrication of vertical organic field effect transistor structures and provides a chance to probe the carrier transport behavior in molecular crystals.38,39 Acknowledgment. We thank the National Science Council (Grant NSC-912120M001003) of the Republic of China for financial support of this work. LA047634U (38) Parashkov, R.; Becker, E.; Hartmann, S.; Ginev, G.; Schneider, D.; Krautwald, H.; Dobbertin, T.; Metzdorf, D.; Brunetti, F.; Schildknecht, C.; Kammoun, A.; Brandes, M.; Riedl, T.; Johannes, H. H.; Kowalsky, W. Appl. Phys. Lett. 2003, 82, 4579. (39) Stutzmann, N.; Friend, R. H.; Sirringhaus, H. Science 2003, 299, 1881.